Lightweight metal pistons have been in use in internal combustion engines for a long time because of their lower weight and reduced inertial forces. In order to protect particularly a first ring groove of such a lightweight metal piston, an aluminium piston, for example, from swelling pressure loads, reinforcements in the form of “ring carriers” are used. The materials from which such ring carriers may be made particularly include iron alloys, for example, that typically have a coefficient of expansion as similar as possible to that of the piston material. However, since for example iron and aluminium alloys have very different heat conducting capabilities, reversing thermal loads can cause strong stresses at the boundary surfaces, and these increase for growing differences between the coefficients of thermal expansion of the two materials, one being used for the piston and the other for the ring carrier. A crack that forms between that ring carriers and the piston typically causes the engine to break down and must therefore be prevented at all costs. The joint between the ring carriers and the piston is usually created with a metallic material in the known in Alfin process, in which the ring carriers is immersed in an aluminium melt until a diffusion layer has formed. Then, this “alfinised” ring carrier is surrounded by the melt of the piston alloy when the piston is cast, and the Alfin bond forms during the subsequent solidification.
Because of the high ignition pressures that prevail in modern diesel engines, practically of the pistons used for this are reinforced at the first ring groove with cast iron ring carriers, usually made from austenite. The trend towards direction fuel injection in petrol engines, combined with rising ignition pressures then also demands more effective wear resistance in the first ring groove than standard piston alloys can provide. At the same time the bond between the lightweight metal of the piston and the ring carrier cast therein is particularly important.
A composite die casting process for manufacturing aluminium pistons for internal combustion engines in which a ring carrier made from metal foam of nickel, copper, iron or alloys thereof having a volume fraction of 3-50% of the piston is infiltrated under a casting pressure of at least 392 bar in a high pressure die casting process to form the bond with the piston alloy is known from DE 34 18 405 C2. A metallurgical bond may be created in a subsequent, multistage heat treatment process, for example solution annealing, aging or the like.
From DE 196 35 326 A1, a method is known from manufacturing a lightweight alloy composite element in which initially a porous composite forming material is held in a hollow space of a casting mould. Then, a molten light alloy is cast in the hollow space of the casting mould by applying a gas pressure, which causes the pores of the porous composite forming material to be impregnated with the molten light alloy. As a result, a composite material section is created that is made from the lightweight alloy and the composite forming material.
In document DE 26 39 294 C2, various highly porous sinter materials with a chromium-nickel base and Cu, Ni, Fe, Ni—Fe-foam materials by infiltration under solidification pressures between 2500 and 1000 bar are described for open porosities from 25-38% for use as ring carriers.